The present invention relates to an image heating device for use in image forming apparatus, such as electrophotographical apparatus, electrostatic recording apparatus or the like and suitable as a fixing device for fixing unfixed images, and to an image forming apparatus using this.
As this kind of image heating device, an image heating device using electromagnetic induction is disclosed in JP 10(1998)-74007 A, JP7 (1995)-295414 A, etc, and is well known.
JP 10(1998)-74007A describes an exciting coil in which a coil is wound around a core, as an exciting means applicable for electromagnetic induction. FIG. 34 is a cross-sectional view showing an image heating device disclosed in JP 10 (1998)-74007 A.
In FIG. 34, reference numeral 310 denotes a coil for generating a high-frequency magnetic field, and 311 denotes a rotatable metal sleeve that generates heat by induction heating. Reference numeral 312 denotes an internal pressure member provided inside the metal sleeve 311, and reference numeral 313 denotes an external pressure member provided outside the metal sleeve 311. This external pressure member 313 is pressed against the internal pressure member 312 via the metal sleeve 311 so as to form a nip portion. The external pressure member 313 is rotated in the direction of the arrow a shown in FIG. 34. The metal sleeve 311 is rotated following the rotation of the external pressure member 313.
A recording paper 314, as a member to be recorded, carrying an unfixed toner image thereon is fed to the nip portion in the arrow direction shown in FIG. 34. Then, the unfixed toner image on the recording paper 314 is fixed by the heat from the metal sleeve 311 and the pressure from both pressure members 312 and 313.
The coil 310 is provided with a plurality of separated winding portions 310a and 310b. These winding portions 310a and 310b are formed by winding a conductive wire around leg portions 315b and 315d of the core 315 via an insulating member (not shown). The core 315 has a plurality of leg portions 315a-315e. Herein, the core 315 is made of ferrite that is a magnetic material, and forms a magnetic path for magnetic flux generated by alternating current applied to the coil 310.
The image heating device disclosed in the above-mentioned JP 10 (1998)-74007A is thought to have the following problems.
Namely, in the configuration of the above-mentioned exciting means, since the conductive wire is wound around the leg portions of the core 315, the position where the conductive wire is placed is limited to the position of the leg portion of the core. Therefore, the degree of freedom of design in placing a conductive wire is limited. Furthermore, it is difficult to place conductive wires in a broader range along the circumferential surface in the circumferential direction of the metal sleeve 311.
On the other hand, JP7 (1995)-295414A describes an excitation means having a configuration in which a conductive coil is placed onto an insulating support body in a curled form. FIG. 35 is a cross-sectional view showing an image heating device disclosed in JP7 (1995)-295414A. FIG. 36 is a perspective view showing a heating coil used in this image heating device.
As shown in FIG. 35, a heating roller 201 is driven to be rotated in the arrow direction while in contact with a pressure roller 202. The pressure roller 202 is rotated following the rotation of the heating roller 201. Furthermore, the pressure roller 202 is pressed to the heating roller 201 and driven to be rotated. A recording paper 203 carrying an unfixed toner image thereon and fed to a place between both rollers 201 and 202 is heated and pressed between the both rollers 201 and 202, and thereby the unfixed toner image on the recording paper 203 is fixed.
A heating coil 204 is provided in a state in which it is embedded in the insulating support body 205. As shown in FIGS. 35 and 36, the heating coil 204 is formed of a narrow conductive film extending along a curved surface of a half cylinder shaped insulating support body 205 and is disposed in a curled shape along the entire width of the insulating support body 205 as a whole. Alternating current is applied to this heating coil 204 from an electric power source for induction heating. Then, due to the alternating current applied to the heating coil 204, alternating magnetic flux is generated so as to excite the heating roller 201. In the heating roller 201, an eddy current is generated that flows in the opposite direction to the direction in which the alternating current flows in the heating coil 204. When the eddy current is generated in the heating roller 201, Joule heat is generated in the heating roller 201, so that the heating roller 201 generates heat.
According to the configuration of the exciting means described in JP 7 (1995)-295414A, as compared with the configuration of the exciting means described in JP10 (1998)-74007A, the degree of freedom of design in placing the conductive wire is less limited, and it is possible to place the conductive wire over a broader range along the circumferential surface in the circumferential direction of the heating roller 201.
However, the image heating device disclosed in JP 7(1995)-295414A has the following problems.
Since the heating coil 204 is formed of a conductive film arranged in a curled form, there is space in which no electric current flows between the circumferentially flowing current. Therefore, as shown by a broken line S in FIG. 35, the magnetic flux passes between the coils to form small loops. In this case, it is not possible to lead the magnetic flux to the heating roller 201 efficiently, thus increasing the magnetic flux that does not penetrate the heating roller 201. Therefore, in order to obtain the electric power necessary for allowing the heating roller 201 to generate heat, a large amount of electric current is required to flow to the heating coil 204. In order to carry a large amount of electric current to the heating coil 204, a component having a large breakdown current is required to be used for the electric power source for induction heating, causing the electric power source for induction heating to be expensive.
Furthermore, conventionally, as image heating devices, for which fixing devices are typical example, contact-heating type devices such as heat roller type devices and belt type devices, generally have been used.
In recent years, due to the demand for shorter warm-up time and reduced energy consumption, the belt type image heating devices capable of reducing the thermal capacity are attracting great attention (see JP 6 (1994)-318001A).
FIG. 37 shows a cross-sectional view of a belt type image heating device, which is disclosed in JP 6 (1994)-318001A. As shown in FIG. 37, an endless rotatable fixing belt 401 is suspended between a fixing roller 402 and a heating roller 403. By heating the heating roller 403 by the use of the heating source H1 located inside the heating roller 403, the fixing belt 401 is heated to a predetermined temperature.
By using the fixing belt 401 having a small thermal capacity, this image heating device is designed to achieve a fixing without offset with less oil applied.
The belt type image heating device including the above-mentioned prior art has advantages of being able to set the thermal capacity of the fixing belt small for shortening the warm-up time, which makes it possible to heat up the fixing belt itself to the predetermined temperature in a short time. However, on the other hand, as the thermal capacity is reduced, the trend for the temperature of the fixing belt to be easily reduced due to the heat removed by the recording paper, etc. when a toner image is fixed becomes larger. Therefore, in order to obtain a reliable fixing, the lowered temperature of the fixing belt should be recovered uniformly to the necessary temperature until the fixing belt arrives again to the fixing portion.
Furthermore, there is another problem in that how the temperature of the fixing belt decreases when the fixing belt passes though the fixing portion varies dependent greatly upon the temperature conditions of the recording paper, the members to be used for pressure means, or the like. Therefore, in order to obtain the stable fixing, regardless of the temperature conditions of the recording paper, the member to be used for pressure means, or the like, that is, even if the manner in which the temperature of the fixing belt decreases changes greatly after the fixing belt passes through the fixing portion, it is necessary to restore the temperature of the fixing belt to the optimum constant temperature when the fixing belt comes again to the fixing portion.
In order to restore the fixing belt to a predetermined temperature stably and uniformly, a configuration of transferring heat from the heat-generating portion to the fixing belt and a configuration of the heat generating portion itself are important. However, in the conventional belt type image heating device, this point was not particularly taken into account.
In the belt type image heating device including the above-mentioned prior art, the thermal capacity of the fixing belt is set to be small in order to shorten the warm-up time, which causes inconsistency in temperature or partially excessive rise in temperature. This is a significant problem in the case of continuously using the recording paper having a smaller width as compared with the size of the depth direction (the direction of the rotation axis of the heating roller 403) of the image heating device shown in FIG. 37. That is, in the portion where the recording paper passes through, the heat is removed increasingly by the recording paper, and therefore the portion must be heated accordingly. However, if the portion where the recording paper does not pass through is heated similarly, the temperature of the portion is raised because the thermal capacity of the heating body (heat-generating roller) is small. Thus, if a large size recording paper (broad-width recording paper) is used in a state in which the temperature is increased abnormally, hot offset may occur.
On the contrary, if the heat generation is limited in order to prevent the hot offset, the temperature of the portion where the heat is removed by the recording paper becomes low, which may lead to the cold offset or an unfixed state.
The present invention has been made to overcome the above-mentioned problems of the prior art. It is an object of the present invention to provide an image heating device capable of obtaining a predetermined amount of heat generation with a small electric current, and an image forming apparatus using the same. Furthermore, it is an object of the present invention to provide a image heating device using a fixing belt and capable of shortening the warm-up time and stably controlling temperatures of the belt, and an image forming apparatus using the same.
In order to achieve the above objects, an image heating device according to a first configuration of the present invention includes a heat-generating member comprising a rotatable body having conductivity, and an exciting coil arranged in opposition to the peripheral surface of the heat-generating member and adapted for allowing the heat-generating member to generate heat with electromagnetic induction, wherein the exciting coil is composed of a bundle of wires having an insulated surface, which are extended in the direction of the rotation axis of the heat-generating member and circumferentially wound along the circumferential direction of the heat-generating member, and the bundled wires extending in the direction of the rotation axis of the heat-generating member are arranged in close contact with each other in at least one place. According to the first configuration of the image heating device, magnetic fluxes, which are generated due to alternating current flowing in the exciting coil, do not pass through between the bundled wires in the area in which the bundled wires are arranged in close contact with each other. Therefore, it is possible to allow the magnetic fluxes to penetrate the heat-generating member efficiently as compared with the prior art. Accordingly, in order to obtain the electric power necessary for allowing the heat-generating member to generate heat, a large amount of electric current is not required to be applied to the exciting coil.
Furthermore, in the first configuration of the image heating device according to the present invention, it is preferable that a larger number of the bundled wires are superimposed at both ends than at the central portion in the direction of the rotation axis of the heat-generating member. With such a preferred configuration, it is possible to heat uniformly a wide range of the heat-generating member in the direction of the rotation axis thereof. Moreover, since the bundled wires superimposed at both ends in the direction of the rotation axis of the heat-generating member are distant from the heat-generating member, an eddy current is not concentrated on this portion and the temperature of this portion is not excessively increased.
Furthermore, in the first configuration of the image heating device according to the present invention, it is preferable that the diameter of the wire is 0.1 mm or more and 0.3 mm or less and the diameter of the bundled wire is 5 mm or less. With such a preferred configuration, since the electric resistance of the bundled wire is small with respect to the high frequency alternating current, the heat generation of the exciting coil can be suppressed. Furthermore, since it is possible to provide the bundled wire with an appropriate thickness, rigidity and durability, the exciting coil can be formed easily.
Furthermore, in the first configuration of the image heating device according to the present invention, it is preferable that the exciting coil has an inductance of 10xcexc H or more and 50xcexc H or less and an electric resistance of 0.5 xcexa9 or more and 5 xcexa9 or less in a state in which the exciting coil is opposed to the heat-generating member. With such a preferred configuration, an exciting circuit can be configured by a circuit element having not so high breakdown current and breakdown voltage, and thus sufficient electric power applied to the heat-generating member and sufficient amount of heat generation can be obtained.
Furthermore, in the first configuration of the image heating device according to the present invention, it is preferable that the image heating device further includes a core made of magnetic material arranged outside the exciting coil. With such a preferred configuration, since all of the magnetic flux at the rear face side of the exciting coil penetrate the inside of the core, it is possible to prevent the magnetic fluxes from leaking out backward. As a result, it is possible to prevent the heat generation due to the electromagnetic induction of the peripheral conductivity material and at the same time to prevent the unnecessary radiation of electromagnetic wave. Furthermore, since the inductance of the exciting coil is increased and the electromagnetic coupling between the exciting coil and the heat-generating member becomes excellent, it is possible to apply larger amount of elastic power to the heat-generating member with same coil current. Furthermore, in this case, it is preferable that the length of the core along the direction of the rotation axis of the heat-generating member is shorter than the length of the heat-generating member in the direction of the rotation axis thereof. With such a preferred configuration, it is possible to prevent the eddy current density at the end face of the heat-generating member from being increased and the heat generation at the end face of the heat-generating member from being excessively increased. Furthermore, in this case, the length of the exciting coil at the outer peripheral portion in the direction of the rotation axis of the heat-generating member is not shorter than the width of a recording material having the maximum width in all the recording materials to be used; and the length of the core in the direction of the rotation axis of the heat-generating member is not shorter than the width of the recording material having the maximum width of all the recording materials to be used. With such a preferred configuration, even if the exciting coil is wound somewhat nonuniformly, it is possible to make the magnetic field reaching from the exciting coil to the heat-generating member to be uniform in the direction of the rotation axis of the heat-generating member. Therefore, it is possible to make the distribution of heat generation of the heat-generating member to be uniform in the portion where the recording material passes through. Thereby, it is possible to make the temperature distribution at the fixing portion uniform, and thus a stable fixing operation can be obtained. Furthermore, it is possible to shorten the length of the heat-generating member in the direction of the rotation axis thereof and the length of the exciting coil in the direction of the rotation axis of the heat-generating member while making the distribution of heat generation of the heat-generating member uniform. As a result, it is possible to realize a miniaturization of the device and at the same time to reduce the cost. Furthermore, in this case, it is preferable that the distance between the end face of the core and the end face of the heat-generating member in the direction of the rotation axis of the heat-generating member is longer than the facing space between the core and the heat-generating member. With such a preferred configuration, since lines of magnetic force radiated from the core toward the end portion of the heat-generating member are not concentrated on the narrow range, it is possible to prevent the induced current from concentrating on the end face and the vicinity of the heat-generating member and to prevent the end portion of the heat-generating member from being excessively heated. Furthermore, in this case, it is preferable that the core has opposing portions opposed to the heat-generating member without sandwiching the exciting coil between the opposing portion and the heat-generating member, and magnetic permeable portions opposed to the heat-generating member via the exciting coil. With such a preferred configuration, since the magnetic fluxes generated by alternating current (coil current) flowing in the exciting coil pass through between the opposing portion and the heat-generating member, most of the magnetic path can be composed of a material having a high magnetic permeability. Therefore, an air portion having a low magnetic permeability in which the magnetic fluxes generated by the coil current passes through is limited to the narrow gap portion between the heat-generating member and the core. Accordingly, the inductance of the exciting coil is increased, and almost all of the magnetic fluxes generated by the coil current can be led to the heat-generating member. As a result, it is possible to obtain an excellent electromagnetic coupling between the heat-generating member and the exciting coil. Thereby, more electric power can be applied to the heat-generating member even with the same coil current. In addition, since the magnetic path is defined by the opposing portion and the heat-generating member, the magnetic circuit can be designed freely. In this case, it is further preferable that the heat-generating member is supported by the support member made of magnetic material, and a space between the support member and the core is twice or more the facing space between the core and the heat-generating member. With such a preferred configuration, most of the magnetic fluxes penetrating the core penetrate the heat-generating member without being led to the support member. Thereby, an electromagnetic energy provided to the exciting coil can be transmitted to the heat-generating member efficiently. At the same time, it is possible to prevent the support member from being heated. Furthermore, in this case, it is preferable that the length between the outermost ends of the magnetic permeable portion along the direction of the rotation axis of the heat-generating member is not longer than the length between the outermost ends of the opposing portion along the direction of the rotation axis of the heat-generating member. With such a preferred configuration, since it is possible to reduce the amount of material for the magnetic permeable portion to be used with the range of the opposing portion defining the range of the heat-generating portion in the direction of the rotation axis of the heat-generating member secured, it can be to make the distribution of heat generation to be uniform with lower cost. Furthermore, in this case, it is preferable that at least a part of the opposing portion is arranged in closer contact with the heat-generating member than the magnetic permeable portion, thereby forming an adjacent portion. With such a preferred configuration, a much greater electric power can be applied to the heat-generating member. Furthermore, in this case, it is preferable that a plurality of adjacent portions are provided band one of the plurality of adjacent portions is located in the center of the winding of the exciting coil. Since a magnetic flux generated by the coil current passes through the center of winding of the exciting coil without fail, by locating the adjacent portion in the center of winding of the exciting coil, the magnetic fluxes generated by the coil current can be led to the heat-generating member efficiently. Furthermore, in this case, it is preferable that at least a part of the core has gaps in the direction of the rotation axis of the heat-generating member. With such a preferred configuration, by changing the arrangement of the core, the distribution of heat generation can be designed freely. Furthermore, even if a cheap and small volume core is used, uniform temperature distribution can be obtained. Furthermore, since heat can be radiated from the gap of the core, and at the same time, the surface area of the core itself becomes large, the radiation of heat can be promoted. Furthermore, in this case, it is preferable that the core has opposing portions opposed to the heat-generating member without sandwiching the exciting coil between the opposing portion and the heat-generating member, and magnetic permeable portions opposed to the heat-generating member via the exciting coil, and the gaps in the magnetic permeable portion of the core are distributed nonuniformly in the direction of the rotation axis of the heat-generating member. Furthermore, in this case, it is preferable that the gap in the magnetic permeable portion of the core is smaller in the end portion than in the central portion in the direction of the rotation axis of the heat-generating member. With such a preferred configuration, it is possible to prevent the deficiency in fixing by making the temperature distribution of the heat-generating member to be uniform. Furthermore, in this case, it is preferable that the core has opposing portions opposed to the heat-generating member without sandwiching the exciting coil between the opposing portion and the heat-generating member, and magnetic permeable portions opposed to the heat-generating member via the exciting coil, and the opposing portions of the core arranged asymmetrically with respect to a center line of the exciting coil in the direction of the rotation axis of the heat-generating member. With such a preferred configuration, it is possible to make the distribution of heat generation in the direction of the rotation axis of the heat-generating member to be uniform with a smaller amount of core. On the contrary, if the amount of core is the same, the distribution of heat generation can be made still more uniform. Furthermore, in this case, it is preferable that the core has opposing portions opposed to the heat-generating member without sandwiching the exciting coil between the opposing portion and the heat-generating member, and magnetic permeable portions opposed to the heat-generating member via the exciting coil, with the gap in the opposing portion of the core smaller than the gap in the magnetic permeable portion of the core in the direction of the rotation axis of the heat-generating member. With such a preferred configuration, since it is possible to reduce the amount of material for the magnetic permeable portion to be used with the length of the core of opposing portion defining the range of the heat-generating portion secured, it can be to make the distribution of heat generation to be uniform with a smaller amount of core material and with lower cost. Furthermore, in this case, it is preferable that the core has opposing portions opposed to the heat-generating member without sandwiching the exciting coil between the opposing portion and the heat-generating member, and magnetic permeable portions opposed to the heat-generating member via the exciting coil, with the opposing portions of the core provided continuously in the direction of the rotation axis of the heat-generating member. With such a preferred configuration, even if gaps are provided in the core of the magnetic permeable portion and are unevenly distributed, the magnetic field reaching from the opposing portion to the heat-generating member can be made uniform in the direction of the rotation axis. Thereby, while the core in the magnetic permeable portion is reduced, the distribution of the heat generation in the heat-generating member in a portion where the recording material passes through can be made uniform, and thus the temperature distribution in the fixing portion can be made uniform. Therefore, a stable fixing operation can be obtained. Furthermore, since the core of the magnetic permeable portion can be reduced while the distribution of heat generation in the heat-generating member uniform, it is possible to achieve the miniaturization of the device and the reduction of the cost. Furthermore, in this case, it is preferable that the heat-generating member is formed in the shape of pipe, and the cross-sectional area of the surface of the inside of the heat-generating member perpendicular to the rotation axis thereof is smaller than the maximum cross sectional area of the core and exciting coil. With such a preferred configuration, it is possible to use the heat-generating member having a small thermal capacity, the exciting coil having a large winding number, and the appropriate amount of ferrite (core) in combination. Therefore, it is possible to apply a larger amount of electric power to the heat-generating member with a predetermined coil current. Furthermore, in this case, it is preferable that a part of the core is divided, thereby forming a movable portion and the movable portion is held movably with respect to the rest portion of the core. Furthermore, in this case, it is preferable that the movable portion arranged outside the region in which a recording material to be used passes through and is allowed to be movable with respect to the remaining portion of the core. With such a preferred configuration, it is possible to prevent the temperature of the member such as a fixing belt, bearing and the like on the end portion from being increased beyond the withstanding temperature due to the excessive increase of the temperature of the region in which the recording material do not pass through. Furthermore, even if a large size recording material is used after small size recording materials are used continuously, since the temperature of the fixing portion is proper, the occurrence of hot offset can be prevented. Therefore, just after the small size recording materials are used, the large size recording material can be used.
Furthermore, in the first configuration of the image heating device according to the present invention, it is preferable that the image heating device further includes a shielding member made of conductive material covering at least a part of a rear face of the exciting coil. With such a preferred configuration, it is possible to prevent a high frequency electromagnetic wave generated from the exciting coil from transmitting to the inside and outside of the apparatus. Thereby, it is possible to prevent electric circuits located at the inside and outside of the apparatus from wrongly operating due to electromagnetic noise.
Furthermore, in the first configuration of the image heating device according to the present invention, it is preferable that the image heating device further includes a cooling means for cooling the exciting coil by air flow.
Furthermore, in the first configuration of the image heating device according to the present invention, it is preferable that the image heating device further includes a heat insulating member for shielding a thermal conduction between the exciting coil and the heat-generating member. With such a preferred configuration, it is possible to cool the exciting coil without cooling the heat-generating member. Furthermore, in this case, it is preferable that the image heating device further includes a core made of magnetic material arranged outside the exiting coil, wherein the length of the exciting coil along the direction of the rotation axis of the heat-generating member is shorter than the length of the heat insulating member along the direction of the rotation axis of the heat-generating member and is longer than the length of the core along the direction of the rotation axis of the heat-generating member. With such a preferred configuration, even in the case where the core is arranged in close to the heat-generating member, the temperature rise of the core can be prevented.
Furthermore, in the first configuration of the image heating device according to the present invention, it is preferable that the image heating device further includes a fixing roller and a fixing belt suspended between the fixing roller and the heat-generating member. Furthermore, in this case, it is preferable that the image heating device further includes a core made of magnetic material arranged outside the exiting coil, wherein the core has opposing portions opposed to the heat-generating member without sandwiching the exciting coil between the opposing portion and the heat-generating member, and magnetic permeable portions opposed to the heat-generating member via the exciting coil, and the length between the outermost ends of the opposing portion along the direction of the rotation axis of the heat-generating member is not longer than the width of the fixing belt. With such a preferred configuration, since the heat-generating member in the portion where heat is not removed by the fixing belt is not heated excessively, the end portion of the heat-generating member can be prevented from being heated excessively.
Furthermore, an image heating device according to a second configuration of the present invention includes a heat-generating member comprising a rotatable body having magnetism and conductivity, and an exciting coil arranged in opposition to the peripheral surface of the heat-generating member and adapted for allowing the heat-generating member to generate heat with electromagnetic induction; wherein the exciting coil composed of a bundle of wires having an insulated surface, which are extended in the direction of the rotation axis of the heat-generating member and circumferentially wound along the circumferential direction of the heat-generating member, and a larger number of bundled wires are superimposed at both ends than at the central portion in the direction of the rotation axis of the heat-generating member.
Furthermore, an image heating device according to a third configuration of the present invention includes a heat-generating member comprising a rotatable body having conductivity; and an exciting coil arranged in opposition to the peripheral surface of the heat-generating member and adapted for allowing the heat-generating member to generate heat with electromagnetic induction; wherein the image heating device further includes a core made of magnetic material arranged outside the exciting coil, and the length of the core along the direction of the rotation axis of the heat-generating member is not shorter than the width of a recording material having the maximum width in all the recording materials to be used.
Furthermore, an image heating device according to a fourth configuration of the present invention includes a heat-generating member comprising a rotatable body having conductivity; and an exciting coil arranged in opposition to the peripheral surface of the heat-generating member and adapted for allowing the heat-generating member to generate heat with electromagnetic induction; the image heating device further includes a core made of magnetic material arranged in a state in which the exciting coil is sandwiched between the core and the heat-generating member, the core has opposing portions opposed to the heat-generating member without sandwiching the exciting coil between the opposing portion and the heat-generating member, and magnetic permeable portions opposed to the heat-generating member via the exciting coil, wherein at least a part of the opposing portion is arranged in closer contact with the heat-generating member than the magnetic permeable portion, thereby forming an adjacent portion, and at least a part of the core has gaps in the direction of the rotation axis of the heat-generating member.
Furthermore, an image heating device according to a fifth configuration of the present invention includes a heat-generating member comprising a rotatable body having conductivity; and an exciting coil arranged in opposition to the peripheral surface of the heat-generating member and adapted for allowing the heat-generating member to generate heat with electromagnetic induction; the image heating device further includes a core made of magnetic material arranged in a state in which the exciting coil is sandwiched between the core and the heat-generating member, the core has opposing portions opposed to the heat-generating member without sandwiching the exciting coil between the opposing portion and the heat-generating member, and magnetic permeable portions opposed to the heat-generating member via the exciting coil, wherein the area of the portion where the opposing portion is opposed to the heat-generating member is larger than the cross sectional area of the magnetic permeable portion perpendicular to the circumferential direction of the heat-generation member. According to the fifth configuration of the image heating device, the electromagnetic coupling between the exciting coil and the heat-generating member becomes excellent, thus improving the efficiency of the heat generation. Furthermore, since magnetic fluxes generated by the coil current are concentrated on the opposing portion of the core, by making the area of the portion where the opposing portion is opposed to the heat-generating member larger than the cross sectional area of the magnetic permeable portion perpendicular to the circumferential direction of the heat-generation member, the amount of heat generation of the heat-generating member in the direction of the rotation axis can be made uniform. Furthermore, it is possible to provide the core with gaps so that the exciting coil has a portion that is not opposed to the core while securing the cross-sectional area where the magnetic fluxes penetrate. Therefore, it is possible to promote the heat radiation from the exciting coil portion and to prevent the magnetic fluxes from leaking outward.
Furthermore, an image heating device according to a sixth configuration of the present invention includes a heat-generating member comprising a rotatable body having conductivity; and an exciting coil arranged in opposition to the peripheral surface of the heat-generating member and adapted for allowing the heat-generating member to generate heat with electromagnetic induction; the image heating device further includes a core made of magnetic material arranged in a state in which the exciting coil is sandwiched between the core and the heat-generating member, wherein a part of the core is divided, thereby forming a movable portion and the movable portion is held movably with respect to the remaining portion of the core.
Furthermore, an image heating device according to a seventh configuration of the present invention includes a fixing belt; a pressure means that is pressed against the fixing belt to form a nip portion on the right side of the fixing belt: a heat-generating roller having at least a part composed of a conductive member and movably suspending the fixing belt; and an exciting coil arranged in opposition to the peripheral surface of the heat-generating roller via the fixing belt and adapted for allowing the heat-generating roller to generate heat by exciting the portion where the heat-generating roller is in contact with the fixing belt. According to the seventh configuration of the image heating device, heat is generated at the portion where the heat-generating roller is in contact with the fixing belt, and the heat is conducted to the fixing belt immediately. Thus, it is not necessary to raise the temperature of the heat-generating roller more than necessary. Consequently, the warm-up time can be shortened.
Furthermore, in the seventh configuration of the image heating device according to the present invention, it is preferable that the width of excitation in the direction in which the fixing belt moves is substantially the same as or not more than the width of the portion where the fixing belt is in contact with the heat-generating roller. With such a preferred configuration, since only the portion that is in contact with the fixing belt is heated in the heat-generating roller, and it is possible to prevent the temperature of the heat-generating roller from being raised abnormally.
Furthermore, in the seventh configuration of the image heating device according to the present invention, it is preferable that the image heating device further includes a temperature detecting means for detecting the temperature, which is arranged in contact with the surface of the heat-generating roller at a portion other than a portion where the heat-generating roller is in contact with the fixing belt; and a control means for controlling an output from the exciting coil in accordance with an output from the temperature detecting means. With such a preferred configuration, it is possible to maintain the temperature of the fixing belt at an optimum temperature.
Furthermore, in the seventh configuration of the image heating device according to the present invention, it is preferable that an exciting current having a predetermined frequency is applied to the exciting coil, and the conductive member of the heat-generating roller has a thickness equal to or larger than the skin depth defined by the material thereof and the predetermined frequency. With such a preferred configuration, at a low temperature, almost all of the induced current can be generated inside the heat-generating roller.
Furthermore, an image heating device according to an eighth configuration of the present invention includes a fixing belt; a pressure means that is pressed against the fixing belt to form a nip portion on the right side of the fixing belt; a heat-generating roller made of magnetic material whose Curie temperature is set to be a predetermined value and movably suspending the fixing belt; a conductive member provided inside the heat-generating roller; and an exciting coil arranged in opposition to the peripheral surface of the heat-generating roller via the fixing belt and adapted for allowing the heat-generating roller to generate heat by exciting the portion where the heat-generating roller is in contact with the fixing belt. According to the eight configuration of the image heating device, since heat is generated at the portion where the heat-generating roller is in contact with the fixing belt, and the heat is conducted to the fixing belt immediately, it is not necessary to raise the temperature of the heat-generating roller more than necessary. As a result, the warm-up time can be shortened.
Furthermore, in the eighth configuration of the image heating device according to the present invention, it is preferable that the conductive member is arranged adiabatically with respect to the heat-generating roller. With such a preferred configuration, heat generated at the heat-generating roller is not conducted to the conductive member easily.
Furthermore, in the eighth configuration of the image heating device according to the present invention, it is preferable that an exciting current having a predetermined frequency is applied to the exciting coil, and the heat-generating roller has a thickness equal to or larger than the skin depth defined by the material thereof and the predetermined frequency.
Furthermore, an image forming apparatus according to the present invention includes an image forming means for forming an unfixed image onto a recording material and having the unfixed image carried thereon; and a fixing device for fixing the unfixed image onto the recording material, wherein an image heating device according to the present invention is used as the fixing device.